The invention relates generally to optical flame detection.
Flame temperature sensors are needed for controlling a wide range of combustion processes. Some combustion processes that require tight control of fuel-to-air ratios for increased fuel burning efficiency and reductions in emission pollution are present in, for example, building heating systems, jet aircrafts, locomotives, and fossil fueled electric power plants and other environments wherein gas and/or steam turbines are used.
Unnecessarily high combustion temperatures can compromise fuel efficiency and increase emission pollution. For example, in a gas turbine designed to emit nine nitrogen oxide (NOx) particles per million (ppm), an increase from 2730xc2x0 F. (1499xc2x0 C.) to 2740xc2x0 F. (1504xc2x0 C.) reduces turbine efficiency by about two percent and increases NOx emissions by about two ppm.
Previous silicon carbide flame detectors such as described in commonly assigned Brown et al., U.S. Pat. No. 5,589,682, issued Dec. 31, 1996, detect the presence of a flame and measure the intensity of the flame""s photon flux over a wide range of wavelengths. The measured intensity, however, does not always correlate to flame temperature, particularly in multiple flame combustors.
In commonly assigned, Brown, U.S. Pat. No. 6,350,988, a continuation in part of aforementioned Brown, U.S. Pat. No. 6,239,434, an optical spectrometer for combustion flame temperature determination includes at least two photodetectors positioned for receiving light from a combustion flame and having different overlapping optical bandwidths for producing respective output signals; and a computer for obtaining a difference between a first respective output signal of a first one of the at least two photodetectors with respect to a second respective output signal of a second one of the at least two photodetectors, dividing the difference by one of the first and second respective output signals to obtain a normalized output signal, and using the normalized output signal to determine the combustion flame temperature.
Commonly assigned, Brown, U.S. Pat. No. 6,350,988, disclosed that gallium nitride, aluminum nitride, and aluminum gallium nitride were promising photodetector materials. More specifically, gallium nitride has a maximum wavelength of absorption of about 365 nanometers (that is, is transparent for wavelengths longer than 365 nanometers); Aluminum nitride has a maximum wavelength of absorption of about 200 nanometers; and a class of alloys of GaN and AlN designated AlxGax-1N are direct bandgap materials with bandgaps variable between the two extremes of GaN and AlN depending on the amount of aluminum in the alloy. The semiconductors of these alloys have optical transitions directly from valance band to conduction band and do not require phonon assistance for transitions (as compared with silicon carbide where such assistance is required). The cutoff in responsivity is therefore sharp and thus provides for high resolution.
For maximum accuracy, dark currents of photodiodes used in combustion flame temperature detection are preferably on the order of less than or equal to about 100 picoamperes per centimeter squared (pA/cm2). Generally gallium nitride and aluminum gallium nitride photodiodes have dark currents on the order of nanoamperes per centimeter squared (xcexcA/cm2) to microamperes per centimeter squared (xcexcA/cm2). Furthermore, yield of good gallium nitride and aluminum gallium nitride photodiodes is relatively low.
It would be desirable to provide a solid state flame temperature sensor for combustion control systems with low dark current and with a flame temperature accuracy within about 20xc2x0 F. (11xc2x0 C.) in the temperature range of about 2500xc2x0 F. (1371xc2x0 C.) to about 3500xc2x0 F. (1927xc2x0 C.).
Briefly, in accordance with one embodiment of the present invention, a solid state optical spectrometer for combustion flame temperature determination comprises: a first photodiode device for obtaining a first photodiode signal, the first photodiode device comprising a silicon carbide photodiode and having a range of optical responsivity within an OH band; a second photodiode device for obtaining a second photodiode signal, the second photodiode device comprising a silicon carbide photodiode and a filter, the second photodiode device having a range of optical responsivity in a different and overlapping portion of the OH band than the first photodiode device; and a computer for obtaining a ratio using the first and second photodiode signals and using the ratio to determine the combustion flame temperature.